The present invention relates to drive devices used for thermal heads in recording apparatus to make thermal records or for thermal heads in display apparatus to form magnetized latent images.
A conventional thermal head includes a number of aligned heater elements which generate heat according to picture data. Thermal pulses generated by the thermal head elements record picture images in a thermo-sensitive recording system or a thermal transfer system or form magnetized latent images in a display device.
A recording or a display apparatus having a thermal head makes a record or display (hereinafter collectively referred to as record) using thermal energy. If the energy becomes either excessive or insufficient, the density of the picture and the picture quality deteriorates the risk of picture quality deterioration increases as the unit printing speed (i.e., the printing repetition period) increases (e.g., repetition periods shorter than 10 m sec.) or as the record density increases.
It, therefore, becomes necessary to modify the picture quality to maintain it in good condition. One known thermal head drive apparatus calculates the status of heat storage in a thermal head to adjust the energy to be applied to the thermal head.
FIG. 1 shows an arrangement of picture data which will be referred to show the calculation of heat storage status in a thermal head for the thermal head drive apparatus and methods discussed herein. Data row L1 includes the data on the line currently being recorded. Data row L2, just above row L1, contains the data recorded immediately prior to the current data row L1. In the same manner, data row L5 contains the data recorded four lines previously. In data row L1, data D.sub.0, meshed in the drawing is referred to as an "aimed data D.sub.0 " and corresponds to the heater element with respect to which printing processing is being performed. Ten reference data D.sub.1 -D.sub.10, shown hatched in FIG. 1, are reference data used for calculating the heat storage condition.
Reference data D.sub.1 and D.sub.2, located adjacent to aimed data D.sub.0, may have relatively great influence to the printing of the aimed data D.sub.0. Reference data D.sub.4, which corresponds to the same heater element on the data row L2 may have the greatest influence to the printing of the aimed data D.sub.0. The reference data which may influence the heat storage for printing aimed data D.sub.0 have different degrees of importance for the calculations of heat storage status depending, for example, on the distance between heater elements and the line printing an interval. The respective reference data D.sub.1 -D.sub.10 are thus weighted before being added to each other to calculate the heat storage state. The weighting is performed, for example, as shown in the following Table 1.
TABLE 1 ______________________________________ REFERENCE DATA WEIGHT ______________________________________ D.sub.1 D.sub.2 7 0 D.sub.3 D.sub.5 4 5 D.sub.4 1 6 0 D.sub.6 D.sub.8 1 7 D.sub.7 1 0 0 D.sub.9 6 0 .sub. D.sub.10 3 6 ______________________________________
The thermal energy needed to print aimed data D.sub.0 is set. In using the numerical values for the heat storage data subjected to weighted addition as described above. That thermal energy may be set by adjusting the width and/or amplitude of the voltage pulse applied to the corresponding heater element of the thermal head.
FIG. 2 shows an example of a conversion relationship between the heat storage state and the applied pulse width in the known apparatus. In FIG. 2, the ordinate indicates width of the pulses applied to the heater element and the abscissa indicates various values for the heat storage state, which are obtained by adding reference data D.sub.1 -D.sub.10 weighted according to Table 1. Each value for the heat storage state corresponds to the heat storage data of the heater element corresponding aimed data D.sub.0. The heat storage state is zero when all the reference data D.sub.1 -D.sub.10 are non-printing data (i.e., white data), and the heat storage state has its maximum value 620 when all the reference data are printing data (i.e., black data).
According to FIG. 2, if the heat storage state for aimed data D.sub.0 is 620, the applied pulse width is 0.3 m sec., the narrowest width, because the heat storage condition is at maximum. If the heat storage state is zero, the applied pulse width is 0.5 m sec., the greatest width, because the heat storage condition is at a minimum.
The applied pulse width is not always determined solely on the basis of the heat storage state in this way, in some devices the applied pulse width is set by referring to the pulse width on the preceding line. In both conventional thermal head drive apparatus, the heat storage state is calculated by referring to past picture data and the applied pulse width is reduced as the heat storage progresses.
In these thermal head drive devices, however, it sometimes becomes impossible to perform sufficient heat storage control when the printing speed increases, which caused excessive heat to be stored in the thermal head. In such devices, the heat stored in the thermal head was temporarily proceeded with the printing speed and has partially taken place in the background (i.e., ground color) portion where printing should not take place, which generated a "foggy" image. In a recording apparatus using a heat transfer recording system, this condition causes, on the other hand, not only the ink at a printing portion, but the ink in surroundings of the portion to be transferred to a recording paper (which is ordinary paper) as if a "tail" was trailed which causes the so-called "tail-trailing" phenomenon.
To eliminate this problem, it became necessary to sample many more reference data as the printing speed increased in order to calculate the present heat storage state. This, however, increased the size of the circuit portion in the thermal head drive device and made the device expensive. Furthermore, sufficient picture density could not be obtained if the energy applied to a recording paper or a thermal recording medium, such as an ink donor film, was suppressed to a low value to avoid a "foggy" image or "tail-trailing."
It is an object of the present invention to eliminate the disadvantages in the prior art thermal head drive devices. Another object of the present invention is a thermal head drive device which performs the proper adjustment of energy applied to heater elements even when the printing speed increases.